1. Field of the Invention
The present invention relates to a method of manufacturing an oxide epitaxially-strained lattice film, and more particularly to a method of manufacturing an oxide dielectric device using an epitaxial dielectric film made of a dielectric material having a perovskite crystalline structure, or the like.
2. Related Art
Recently, storage devices using ferroelectric thin films (ferroelectric memory devices) as storage mediums have been developed, and some of them have been brought into practice. Ferroelectric memory devices have the advantages that they are nonvolatile, therefore maintain the storage even after removal of power, quick in spontaneous polarization reversal, when having a sufficiently thin film thickness, and are therefore available for quick write and read equivalent to those of DRAM. Additionally, since each one-bit memory cell can be made of a single transistor and a single ferroelectric capacitor, they are suitable for realization of larger capacities.
Ferroelectric thin films for use in ferroelectric memory devices are required to have properties: remanent polarization being large, dependency of remanent polarization upon temperature being small, remanent polarization being maintained for a long time (retention), among others.
Currently, lead zirconate titanate (PZT) is mainly used as a ferroelectric material. However, regardless of its high Curie temperature (300xc2x0 C. or higher) and large spontaneous polarization, its major component, Pb, is liable to disperse and vaporize at relatively low temperatures (for example, 500xc2x0 C.), and this material is estimated to be difficult to cope with miniaturization.
Under the circumstances, the Inventors found and now recognize that the c-axis length of BST can be artificially controlled by selecting strontium titanate (SrTiO3, which may be called STO hereinbelow) single crystal as the substrate, strontium ruthenate (SrRuO3, which may be called SRO hereinbelow), for example, as the lower electrode, and a material having a slightly larger lattice constant than that of SRO, such as barium strontium titanate (BaxSr1-xTiO3, which may be called BST hereinbelow) as the dielectric material, and by epitaxially growing all, employing a deposition method that is more effective in preventing misfit-dislocation in the process of deposition of film by RF magnetron sputtering such that the epitaxial effect maintains BST as a strained lattice even when the film is relatively thick as large as 200 nm or more (see Japanese Patent Publication No. 2878986). As a result, it has been confirmed that, by using BST having a Ba-rich composition, it is possible to realize a ferroelectric thin film quite desirable as ferroelectric memory (which may be called FRAM hereinbelow), which can shift the ferroelectric Curie temperature toward a higher side, exhibit a large remanent polarization at room temperatures and maintain a sufficiently large remanent polarization even when the temperature is raised to about 85xc2x0 C. Thus, FRAM can be made by using a thin-film capacitor having an epitaxially grown, ferroelectric thin film, and its practical availability is expected.
However, through various researches about methods of making epitaxially-strained lattice ferroelectric thin films, the Inventors have come to the realization that serious difficulty still exists in uniformly making a large dimension of epitaxial ferroelectric film having good crystalline property and ferroelectric property.
The xe2x80x9cstrainxe2x80x9d in the term xe2x80x9cstrained latticexe2x80x9d used herein has a meaning different from the xe2x80x9cstrainxe2x80x9d naturally introduced into the lattice as a result of appearance of ferroelectricity. For example, barium titanate (BaTiO3) has a cubic structure of a paraelectric substance at temperatures higher than the Curie temperature (approximately 130xc2x0 C.), and all of its a-axis, b-axis and c-axis are 4.01 xc3x85. At the Curie temperature, however, that structure changes to a tetragonal structure due to ferroelectric phase transformation, which results in contraction of the a-axis and the b-axis by approximately 0.005 xc3x85 and expansion of the c-axis by 0.01 xc3x85. This type of change is often called a distortion caused by ferroelectricity, but it is different from the strain in the context of the present invention. The strain used in the present invention means a rather artificial strain caused by a restriction that a crystal lattice having such a naturally introduced distortion receives from the substrate while epitaxial growth progresses.
[Prior Art 1]
FIG. 1 is a layout diagram of a well-known parallel-flat RF sputtering apparatus. Numeral 101 denotes a substrate holder, 102 refers to a substrate, and 103 to a cathode. The cathode 103 is made up of a target 105, backing plate 106, magnet 107 and yoke 108. A magnetic field as shown by lines of magnetic force 109 is generated by the magnet 107. Using that apparatus, using BaTiO3 ceramic as the target for a ferroelectric film and SrRuO3 ceramic as the target for an upper or lower electrode, using a single-crystal SrTiO3 substrate as the substrate 102, setting the substrate temperature at 600xc2x0 C., supplying Ar and O2 by the ratio of 4:1 to adjust the total pressure at 0.25 Pa, the SrRuO3 lower electrode, BaTiO3 ferroelectric film and SrRuO3 upper electrode were stacked in this order to the thicknesses of 30 nm, 40 nm and 30 nm, respectively, on the SrTiO3 substrate. RF power supplied was 300 W for all of the targets. With the BaTiO3 ferroelectric film obtained, its lattice constant was measured, and the relations between c-axis lengths and substrate positions were collected as shown in FIG. 2. Substrate positions were shown by angles xcex8 from the region of the target to be sputtered, which is shown in FIG. 1 and called an erosion region 104. As shown in FIG. 2, epitaxial growth did not occur at the substrate position opposed to the erosion region, i.e., the position where xcex8 is near 0, and its c-axis value could not be measured. On the other hand, at positions distant from the position opposed to the erosion region, where xcex8 is larger than about 15 degrees, epitaxial growth occurs, and the c-axis value increases from the original bulk c-axis value. That is, lattice mismatch with the substrate is maintained without being relaxed by crystal defects, and a strained lattice is made. As shown in FIG. 2, the c-axis exhibited the maximum value, and a very strong ferroelectricity was observed near the portion where the angle xcex8 from the erosion region was about 20 degrees.
As reviewed above, if a typical parallel-flat RF sputtering apparatus is used, then the crystalline property is damaged at the portion opposed to the erosion region, and strained lattices are made merely in offset regions distant by a certain value from the position opposed to the erosion region because of improvement of the crystalline property. Therefore, ferroelectric capacitors having good ferroelectric characteristics cannot be made uniformly all over a substrate with the existing apparatus alone.
That phenomenon is known as damaging effect of oxygen negative ions, which occurs during sputtering of oxides (see, for example, D. J. Kester and R. Messier; J. Vac. Sci. Technol., A4-3(1986), 496 or K. Tominaga, N. Ueshiba, Y. Shintani and O. Tada; Jap. J. Appl. Phys., 20-3(1981), 519). This is shown in a schematic diagram of FIG. 3. When RF power 306 is supplied to the target 301, the target is negatively charged with respect to the plasma potential, and a strong electric field 307 is generated in the plasma sheath portion. Ar positive ions 304 in the plasma 303 are accelerated by the electric field 307 of the plasma sheath portion, hit the surface of the target 301, thereby bash out atoms forming the target by a sputtering action and can stack them on the substrate 302 located in confrontation with the target. However, in case the target is an oxide like BaTiO3, sputtered oxygen is liable to become negative ions 305, and since these oxygen ions are negatively charged, they are accelerated by the electric field 307 of the plasma sheath portion toward the substrate away from the target, rather sputter the substrate plane, and damage the crystal grown on the substrate. This is the most possible assumption. Since those oxygen negative ions are accelerated vertically of the target, they seriously damage the portion opposed to the erosion region. On the other hand, particles sputtered from the target spread over a relatively wide area, and it is considered that a ferroelectric film having a good crystalline property stacks on the region distant from the erosion-opposed portion.
To prevent damage by oxygen negative ions produce by sputtering of an oxide, there are roughly two groups of methods. Those methods proposed heretofore are explained below.
[Prior Art 2]
One of those methods is to increase the sputtering gas pressure. This method is configured to increase the gas pressure so as to make oxygen negative ions accelerated by an electric field hit the gas and disperse, thereby to decrease the kinetic energy below the threshold value causing damage to the crystal grown.
The Inventors conducted an experiment in which only the sputtering gas pressure was raised by using the same apparatus and the material as those of the [Prior Art 1]. With the BaTiO3 ferroelectric film obtained, its lattice constant was measured, and the relations between c-axis lengths and substrate positions were collected as shown in FIG. 4. As the gas pressure is raised from 0.25 Pa, the epitaxially grown region gradually expands, and the position where the c-axis length exhibits the peak value approaches the erosion region, and in case of 10 Pa, it exhibits the maximum value just above the erosion region. When the gas pressure is further raised to 25 Pa, epitaxial growth takes place in the entire region; the c-axis does not extend substantially any more.
Increasing the gas pressure in this manner gives certain effects in preventing the damaging effect of oxygen negative ions and enabling epitaxial growth over the entire area. However, even when the gas pressure is optimized, a considerable distribution of the c-axis length remains. Namely, there remains the characteristic that the c-axis length reaches the maximum value at a position at a specific distance from the position opposed to the erosion region, and decreases as moving away from the position. Additionally, changes of the ferroelectric property are confirmed to larger than changes of c-axis value through measurement. Therefore, although this technique gives a certain effect, it does not satisfy the required specification because of fluctuation of the characteristics when manufacturing a ferroelectric memory uniformly over a semiconductor substrate with a large area.
[Prior Art 3]
The second method used heretofore to prevent damage by oxygen negative ions is off-axis sputtering in which the substrate is located offset from vertically above the target surface from which the oxygen negative ions are radiated. This method relies on locating the substrate aside from the target and stacking only sputter particles diagonally radiated from the target.
An example of off-axis sputtering apparatus is shown in FIG. 5. Two targets 503 are located face to face, and a substrate 502 is disposed at the center between the two targets to incline by 90 degrees from the target planes. The substrate 502 is fixed on a substrate holder 501, and the targets 503, in combination with backing plates 504 and magnets 505, form cathodes. A magnetic field as shown by lines of magnetic force 506 is generated by the magnet 505. An experiment was conducted under the same conditions as those of the foregoing Prior Art 1 except for using that apparatus. With the BaTiO3 ferroelectric film obtained, its lattice constant was measured, and the relations between c-axis lengths and substrate positions were collected as shown in FIG. 6. It was confirmed from FIG. 6 that epitaxial growth occurred wherever of the substrate when the apparatus of FIG. 5 was used. However, extension of the c-axis of the obtained film was small, and this demonstrates a structure was obtained in which no strained lattice was formed and the strain was relaxed at the boundary. As reviewed above, locating the targets and the substrate at off-axis positions gives certain effects in preventing the damaging effect of oxygen negative ions and enabling epitaxial growth over the entire area. However, there still remains the problem that a sufficient c-axis length cannot be obtained, that is, strained lattices are not formed sufficiently.
If conventional parallel-flat RF sputtering is used for making epitaxially strained lattices of an oxide, good strained lattices cannot be made because oxygen negative ions seriously damage the portion opposed to the erosion region of the target. In case a high gas pressure atmosphere is introduced into a parallel-flat RF sputtering apparatus or an off-axis sputtering apparatus is used for the purpose of preventing damage by oxygen negative ions, although epitaxial growth is ensured over the entire area of the substrate, from the viewpoint of strained lattices, it is not possible to introduce required strain uniformly over the entire area of the substrate and prevent that the strain is relaxed at the boundary.
The invention has been made to solve the problems discussed above, and to make strained lattices of an oxide uniformly all over the substrate, particularly make strained lattices of a ferroelectric material such as BaTiO3 all over the substrate and thereby enable its use as an excellent ferroelectric capacitor of a semiconductor memory device.
To attain the object of the invention, the Inventors repeated reviews of Prior Arts, and as a result of new trials, first got the following knowledge.
That is, according to the knowledge the Inventors obtained, in order to ensure that an excellent strained lattice of an oxide is made by sputtering, mere prevention of damage by oxygen negative ions is not sufficient, but it is indispensable to simultaneously irradiate particles having an energy necessary for fabrication of the strained lattice but lower than the energy of damaging particles.
Additionally, unlike the foregoing [Prior Art 2], when the gas pressure is raised progressively, a substrate region where particles having an energy as high as causing damage gradually moves to just above the erosion region, and outside of it, there is a substrate region where particles are adequately scattered and have an energy required for growth of the strained lattice. Thus the Inventors found that an oxide strained lattice with a well-extended c-axis appeared in a specific substrate region, depending upon the gas pressure.
Based on the knowledge first obtained, the Inventors found a method of uniformly fabricating epitaxially strained lattices of an oxide over a substrate having a larger area, as explained below.
That is, according to the invention, there is provided a method of manufacturing an epitaxially-strained lattice film of an oxide on a substrate by sputtering in vapor phase at least a part of film components from a target onto the substrate, comprising:
(1) preventing damage to the strained lattice film by oxygen negative ions; and
(2) stacking the epitaxially strained lattice film while applying RF power to the substrate in order to maintain the DC bias potential of the substrate between +5V and xe2x88x9230V.
According to the first aspect of the invention, there is provided a method of manufacturing an epitaxially-strained lattice film of an oxide on a substrate by sputtering in vapor phase at least a part of film components from a target onto the substrate, comprising:
(1) using a sputtering apparatus having a plurality of targets opposed to the substrate, shield plates located vertically above surfaces of the targets in position fixed with respect to the targets, and a mechanism for relatively rotating the entirety of the targets and the shield plates, and the substrate; and
(2) stacking the epitaxially strained lattice film by sputtering while applying RF power to the substrate in order to maintain the DC bias potential of a substrate holder between +5V and xe2x88x9230V.
According to the second aspect of the invention, there is provided a method of manufacturing an epitaxially-strained lattice film of an oxide on a substrate by sputtering in vapor phase at least a part of film components from a target onto the substrate, comprising:
(1) using an offset or off-axis sputtering apparatus in which the substrate is not positioned in a region vertically above the target plane; and
(2) stacking the epitaxially strained lattice film by sputtering while applying RF power to a substrate in order to maintain the DC bias potential of the substrate holder between +5V and xe2x88x9230V.
According to the third aspect of the invention, there is provided a method of manufacturing an epitaxially-strained lattice film of an oxide on a substrate by sputtering in vapor phase at least a part of film components from a target onto the substrate, comprising:
(1) using a parallel-flat sputtering apparatus in which the substrate and the target are positioned face to face, and when the gas pressure during deposition is P (Pa) and the distance between the substrate and the target is L (mm), maintaining the product of P and L not smaller than 500; and
(2) stacking the epitaxially strained lattice film by sputtering while applying RF power to a substrate in order to maintain the DC bias potential of the substrate holder between +5V and xe2x88x9230V.